Wastewater streams from industries using large amounts of processing water, such as the textile industry in its various dyeing operations, often generate large quantities of aqueous wastewater which can be fairly highly alkaline in content. Before such streams can be discharged either to a municipal treatment plant or to the environment, they must be neutralized. Such neutralization has frequently been carried out using strong mineral acids; however, in recent years carbon dioxide, which forms carbonic acid when dissolved in water, has frequently been used for neutralizing such alkaline wastewaters.
As depicted in U.S. Pat. No. 4,743,405, issued May 10, 1988, apparatus is shown for injecting gaseous CO.sub.2 into a flowing alkaline liquid. More particularly the gaseous CO.sub.2 is supplied to an enclosure or compartment which surrounds a section of a conduit through which the alkaline stream is flowing and which serves as a manifold to supply gaseous CO.sub.2 through a plurality of hollow needles which project into the flowing stream. In such systems of this general type for the neutralization of alkaline streams, it has been conventional to inject CO.sub.2 in the gaseous form. Because it is common to store CO.sub.2 in the liquid state, usually at about 300 psig and 0.degree. F., it is necessary to supply equipment to vaporize the liquid CO.sub.2 before it can be delivered to such a gas injection apparatus.
As far back as 1928, as shown in U.S. Pat. No. 1,655,816 to Josephson, various attempts were made to inject liquid CO.sub.2 for carbonation purposes. More recently, U.S. Pat. No. 4,068,010, issued Jan. 10, 1978, shows a method for liquid CO.sub.2 injection to simultaneously carbonate and cool a flowing aqueous stream to produce a carbonated beverage or the like. Such an application requires fairly high quantities of carbon dioxide, and as a result, the creation of water ice crystals is a potential problem which is taken into account by the injection of pumped liquid CO.sub.2 at fairly high pressures, e.g. between 300 and 2000 psi, through a spring-loaded valve.
Systems of the above type have not proved to be particularly useful for relatively fine control, and work continued to develop systems that will operate efficiently at relatively low pressures.